Crystallisation entropy

This thermodynamic behaviour is consistent with stress-induced crystallisation of the rubber molecules on extension. Such crystallisation would account for the decrease in entropy, as the disorder of the randomly coiled molecules gave way to well-ordered crystalline regions within the specimen. X-Ray diffraction has confirmed that crystallisation does indeed take place, and that the crystallites formed have one axis in the direction of elongation of the rubber. Stressed natural rubbers do not crystallise completely, but instead consist of these crystallites embedded in a matrix of essentially amorphous rubber. Typical dimensions of crystallites in stressed rubber are of the order of 10 to 100 nm, and since the molecules of such materials are typically some 2000 nm in length, they must pass through several alternate crystalline and amorphous regions. 

FIG. 20.4 Enthalpy and entropy levels of monomer and polymer in different physical states. Symbols g = gaseous I = liquid a = amorphous c = crystalline s = dissolved vap = vaporisation (condensation) m = melting (crystallisation) o = standard state (25 °C, 1 bar). 

The presence of the metal or insulator does not only add the molecule-substrate interaction as a formative influence, but can also alter the effective intermolecular interactions. For example, whereas the crystallisation of bulk tetraeene is governed by the attractive interaction between molecules in a particular relative orientation, the surface-confined molecules (on Ag( 111)) repel each other. The modification of the effective intermolecular interaction may originate both from substrate-mediation and from the intrinsically anisotropic molecular interaction potentials. As the possibly entropy-driven ordering of tetraeene on Ag(lll) shows, the modified interactions may introduce new ordering mechanisms at the interface. 

In some cases crystallisation sequences are observed where a first formed phase transforms after extended reaction times to other phases. This kind of behaviour is in accord with Ostwald s law, which states that under kinetically controlled conditions, the first phases to form will be those with higher entropy, and these may transform towards the thermodynamically most stable phase via phases with progressively lower entropy and lower free energy. The sequential synthesis of zeolite Na-Y and then the denser zeolite Na-P from the same... 

If rubber is investigated under conditions giving rise to increased intermolecular cohesion e. g, if it partly crystallises on cooling or at high extensions) the theoretical treatment becomes more complicated and extremely difficult since then, besides entropy relations, energetic factors also come into the picture. The mechanical behaviour then also becomes more and more analogous to that of fibrous proteins and cellulose where intermolecular cohesion plays an intrinsic part. 

Rubber thus frozen in the extended state does not recover elastically upon release of stress it has a permanent set . In other words, retraction is blocked by cohesive forces On warming up, however, retraction occurs at once Under ordinary conditions, at room temperature, however, the lattice forces of the crystallites formed upon extension are not strong enough to prevent the rubber from retraction. The entropy factor then surpasses the work of crystallisation. 

Hel is a fairly normal liquid, remarkable only for its low temperature and density. The transition to Hell that occurs at the lambda temperature is not an ordinary, first order, phase change since there is no abrupt change in specific volume and specific entropy. Nor is it a formation of molecules or loosely bound complexes (no Raman effect). Further, X-ray studies show no kind of pseudo-crystallisation. 

A polymer melt consists of randomly coiled and entangled chains. This gives much higher entropy than if the molecules are the form of extended chains since there are many more conformations available to a coil than for a fully extended chain. The higher value of S leads to lower value of G. Now if the melt is cooled to a temperature below the melting point of polymer, Tm, crystallisation may occur. There is a high degree of order in... 

---